The light sensitive retina, an extension of the central nervous system (CNS) located at the back of the eye, is essential for sight but vulnerable to both acute injury and chronic disease. As with other CNS regions, neurons in the adult retina are not replaced when lost, and the retinal ganglion cells (RGCs) that aggregate and relay visual information to the brain are particularly vulnerable to dysfunction and death in glaucoma, resulting in irreversible vision loss1,2. Glaucoma is a leading cause of blindness worldwide3, yet despite the devastating impact on affected individuals and the overall costs to society, much remains unknown about how the early stages of the disease contribute to RGC loss4. The role of glia is a major area of investigation in glaucoma pathophysiology, as these non-neuronal support cells are essential for RGC survival5 but undergo phenotypic changes in disease that may diminish beneficial behavior6 or even drive the adoption of deleterious phenotypes7. Although the term glia encompasses a range of specialized cell types throughout the CNS8, these can be broadly divided into two categories - those that share a developmental lineage with neurons and provide trophic, energetic, and structural support9, and those with a myeloid origin, which perform specialized immune surveillance and response tasks10. Representatives of both categories - astrocytes and Müller cells in the former, microglia in the latter - populate the retina5 and undergo major changes in glaucoma that raise significant questions about their role in disease progression and RGC survival11.
Efforts to answer these questions are hindered in part by intrinsic characteristics of retinal glia that present challenges to both in vivo and in vitro investigation. Unlike neurons, they are relatively silent electrically, making approaches such as ERG that enable functional assessment of RGCs and photoreceptors in vivo unsuitable for investigating changes in glial function and behavior. And while inducible12 and spontaneous13 models of glaucoma are available, much remains unknown about changes in glia at the earliest time points in the disease, which may precede detectable RGC loss, a problem compounded by the variable penetrance of disease state in many of these models. Furthermore, evidence from both clinical and experimental glaucoma suggests contributions from peripheral immune cells that can be difficult to disambiguate from those of microglia, despite indicators that they may act in different 'directions' to influence disease progression14,15.
Challenges also abound in studying retinal glia in vitro. Although isolation and culture of astrocytes16,17 and microglia18 from the brain are well established, recent work highlights extensive glial heterogeneity between CNS regions, especially in astrocytes19, and phenotypes present in one region may not be present in glia from another, such as the retina20,21. However, directly isolating retinal astrocytes and microglia for study is particularly challenging, as both cell types are relatively sparse - each making up less than 1% of the estimated 6.5 million cells in the mouse retina22,23,24. Furthermore, unlike neurons, glia are highly plastic and rapidly adapt to dramatic changes in their surroundings16,18,25; as a result, behaviors observed in these cells in vitro may represent specific adaptations to their new environment rather than phenotypic patterns that would be typically seen in health or disease. Given the limitations of both in vivo experimentation and primary culture of retinal glia, we have sought an intermediate approach - retinal explants - in which glia, RGCs, and other elements of the retina are preserved in situ in an ex vivo context. Relative to in vivo models, this approach offers an abbreviated experimental time course26, enables direct experimental manipulation of retinal glia27, and avoids the potentially confounding influence of peripheral immune cell infiltration14. Conversely, unlike primary cell culture, there is minimal disruption of the extracellular environment, and glia remain intact and morphologically recognizable, obviating the challenges associated with essentially regrowing and identifying these cells after enzymatic and mechanical disruption16,25.
Relative to previously described retinal explant methodologies, this approach emphasizes a focus on technical reliability, reproducibility, and maximizing the 'user-friendliness' of the approach to improve accessibility26,28. In personal communications with other researchers, we found that the technical challenges associated with handling the live retina present a major hurdle to many looking to utilize explants, whereas maintenance of the explanted retina in culture was relatively straightforward for groups with appropriate cell culture facilities. Therefore, this protocol includes a number of innovations intended to reduce the learning curve associated with retina isolation and allow researchers to more rapidly begin collecting experimental data. Finally, although we have placed special emphasis on the potential of this explant model for investigating the behavior of retinal glia and characterize it primarily with immunofluorescence microscopy, other retinal cell types and structures are largely conserved as well, and the explanted retinas remain amenable to a wide range of additional investigatory techniques.